The present invention relates to a method and an apparatus for dispensing or atomizing small amounts of liquids having a tendency to solidify or dry out when exposed to atmospheric air.
In chemical, pharmaceutical and biomedical applications, such as chemical analysis of liquid samples and medical coating applications, there is a trend to use an apparatus to dispense and atomize a liquid also referred to as process liquid that tend to solidify and may also be of high viscosity. In its simplest form, such an apparatus will include means to supply the process liquid and a device for atomizing or dispensing the liquid, using for example ultrasonic, electrostatic and/or pneumatic means.
It is frequently desirable to employ an apparatus comprising a nozzle that is of a relatively small orifice diameter to disintegrate small liquid amounts. However, many liquids tend to solidify or dry out when exposed to atmospheric air, leaving behind a film of dried product on the sides of the nozzle orifice, which will tend to narrow the orifice, particularly as the build-up increases over time. As the dimension of the nozzle orifice is critical to produce the desired spray pattern, the spray pattern typically becomes more irregular and has larger and more unevenly sized liquid particles when the surface of the orifice become coated with dried product. In case of a dispensing operation, slight depositions at the orifice will generally adulterate the drop volume and result in variations of the volume of liquid supplied. Thus, the process will not be repeatable and requires time-consuming maintenance for readjusting the drop volume and eliminating the nozzle built-up.
It is known to perform preventative routine maintenance or a specific operational sequence including a start-up or a purging sequence to minimize nozzle built-up. However, cleaning cycles involving flushing the fluid line with a cleaning liquid and particle filters do not completely eliminate nozzle built-up. There is a risk that solid matter is still present, may get loose and leads to inhomogeneous droplet sizes and nozzle clogging. Even relatively short standstill periods may result in gas inclusions and crystallization or sedimentation of particles or components, which may cause malfunction or clogging of the apparatus and result in time-consuming cleaning procedures.
Other attempted solutions include covering the orifices using mechanical means when inactive. In addition, it may be required to dismount or move the nozzle during maintenance, which will often result in incorrect alignment of the nozzle in relation to the substrate.
In coating processes, such as drug coating of medical implants, it is necessary to accurately control the deposition of material on the object in order to ensure a homogeneous coating and a consistent coating weight. However, in prior art systems nozzle built-up and clogging often leads to poor performance of the atomizing or dispensing device resulting in expensive maintenance and coating defects. In addition, starting and stopping a spraying cycle, which may be desirable to reduce waste of hazardous and/or expensive material, may not be possible due to nozzle built-up and clogging. Thus, nozzle clogging can give rise to both quality and productivity problems.